Carrier projectiles, also termed carriers, typically include projectile-shaped bodies with an open aft end for receiving cargo, and a base closure for retaining cargo. The cargo is deployed by a time-fuzed ejection charge which severs a base closure joint to expel the cargo during projectile flight.
Current thickwall carrier designs join an engraving band to the wall of the projectile body. Cargo is keyed to the body wall as a means of fixing the cargo to the projectile. The design is reliable as it provides a straightforward in-bore torquing path from the banded body to the cargo via a mechanical interface.
A new thinwall carrier projectile concept arose from a desire to provide a cargo compartment capable of packaging a cargo of maximum feasible diameter within a given projectile caliber. The thinwall concept departs from that of thickwall carrier projectiles in that its cargo is keyed to a projectile base member, using a castellated interface or the like, rather than being keyed to a wall of a projectile body. This keying method, or constraint, presents design difficulties because the projectile base, in aggregate with its keyed cargo, is now a major inertial member of the projectile. Accordingly, both the projectile body and base require a substantial torquing means.
Candidate methods for accommodating the thinwall concept for its in-bore axial and angular acceleration environments include:
(1) An engraving band metallurgically bonded to a carrier body, a base keyed with an abutting cargo member, the base joined to the carrier body with threads,
(2) An engraving band bonded to a base (instead of a carrier body), the base keyed to a cargo member, the base threaded to a carrier body.
In each case, an expelling charge is used to sever the threaded joint to permit expulsion of cargo. Therefore, joint design is cooperatively limited in retention strength.
In the first design, wherein the carrier body is banded, a friction interface between carrier body and base is relied upon to torque the base including its keyed cargo.
The second design, which uses a banded base, relies on an enhanced friction interface consisting of a knurled body face and knurled abutting base shoulder to torque the carrier body via the base.
Although both the above design arrangements may prove capable of withstanding in-bore and handling environments, there is a risk that neither will. In both methods, friction interfaces are required to transmit torque to major inertial members of the projectile. Since these friction interfaces do not provide a positive locking method, performance reliability is at risk.
The risk is greatest in worn tube firings. In one type of wear, tube rifling is worn severly at its origin. A gun-fired projectile thus accelerates freely before its band engages the tube rifling. Upon engagement of the band with the rifling, the projectile is subjected to a transiently high angular acceleration even before the band is fully engraved and while axial acceleration is low. Under these conditions effectiveness of friction interfaces are impaired relative to torque demands.
A third possible design approach is to employ a banded carrier body, a spline and groove interface between body and base, and a plurality of shear pins spaced along a circumference of the body and radially emplaced through the body and through the body-base interface. This method is capable of withstanding in-bore, rough handling, and transportation environments. However, the method is costly due to a great number of shear pins and matched holes needed for adequate base and cargo retention. In addition, where it is necessary to access cargo for reasons of surveillance, rework, or replacement of limited shelf life componentry, disassembly of the projectile closure is difficult.